1. Field of the Invention
The present invention relates to an image processing apparatus for recording and displaying an image, and more particularly to improvement in the resolution and tonality of a half-toner image recorded or displayed by a laser beam printer or an ink jet printer.
2. Description of the Prior Art
Many methods to produce a half-tone image in a digital printer have been proposed. Examples thereof are a dither method and a density pattern method. These methods have been used in many fields for the reasons that:
(1) The half-tone image can be displayed by a digital display device, PA0 (2) a hardware configuration of the apparatus is easy, and PA0 (3) satisfactory image quality is attained.
Specifically, as shown in FIGS. 1A and 1B, each of picture cells 8 of an input image is compared with a corresponding one of elements of a threshold matrix 5 to determine if it is white or black by threshold comparison in order to selectively display dots on a display screen 6.
FIG. 1A illustrate the dither method in which each of the picture cells 8 of the input image corresponds to one element of the threshold matrix 5. FIG. 1B illustrates the density pattern method in which each of the picture cells 8 of the input image corresponds to all elements of the threshold matrix 5. Thus, in the density pattern method, each of the picture cells of the input image is displayed by a plurality of cells on the display screen 6.
A difference between the dither method and the density pattern method resides in that one picture cell of the input image corresponds to one element of the threshold matrix in the former method while it corresponds to all elements of the threshold matrix in the latter method, and it is not an essential difference. An intermediate method has also been proposed, in which one picture cell of the input image corresponds to a certain number (for example, 2.times.2=4 in FIG. 1B) of elements of the threshold matrix.
Accordingly, there is no essential difference between the density pattern method and the dither method. The density pattern method and the intermediate method are hereinafter collectively referred to as the dither method. In such a dither method, the threshold matrix can be prepared in many methods. However, a method or apparatus for allowing a high quality of image output has not been proposed.